Not to mention that, despite having done nothing wrong, you can still be blamed and suffer the consequences. Imagine that company did notice, and the person who sent the payment went into a panic. They call their bank, that bank calls your bank, they put a hold on your entire account, and now your payroll, bills, leases, etc. all start bouncing and you can't accept payments from other clients to cover anything in the short term.

Now you've done nothing wrong, maybe even haven't noticed yet, and suddenly they kill your business overnight.